The x-ray tube has become essential in medical diagnostic imaging, medical therapy, and various medical testing and material analysis industries. Typical x-ray tubes are built with a rotating anode structure that is rotated by an induction motor comprising a cylindrical rotor built into a cantilevered axle that supports the disc shaped anode target, and an iron stator structure with copper windings that surrounds the elongated neck of the x-ray tube that contains the rotor. The rotor of the rotating anode assembly being driven by the stator which surrounds the rotor of the anode assembly is at anodic potential while the stator is referenced electrically to ground. The x-ray tube cathode provides a focused electron beam which is accelerated across the anode-to-cathode vacuum gap and produces x-rays upon impact with the anode target. The target typically comprises a disk made of a refractory metal such as tungsten, molybdenum or alloys thereof and the x-rays are generated by making the electron beam collide with this target, while the target is being rotated at high speed. High speed rotating anodes can reach 9,000 to 11,000 RPM.
Only a small surface area of the target is bombarded with electrons. This small surface area is referred to as the focal spot, and forms a source of x-rays. Thermal management is critical in a successful target anode, since over 99 percent of the energy delivered to the target anode is dissipated as heat, while significantly less than 1 percent of the delivered energy is converted to x-rays. Given the relatively large amounts of energy which are typically conducted into the target anode, it is understandable that the target anode must be able to efficiently dissipate heat. The high levels of instantaneous power delivered to the target, combined with the small size of the focal spot, has led designers of x-ray tubes to cause the target anode to rotate, thereby distributing the thermal flux throughout a larger region of the target anode. There are various techniques for distributing thermal flux, for example, faster rotation speeds or greater target anode diameters, that allow for decreasing the thermal energy at any given location along the focal track.
However, there is a practical limitation regarding a maximum speed at which the target anode can be rotated, and in the size of practical target anode diameters. The materials of the target anode will eventually shatter at certain speeds and larger diameters.
Operating conditions for x-ray tubes have changed considerably in the last two decades. U.S. Pat. No. 4,119,261, issued Oct. 10, 1978, and U.S. Pat. No. 4,129,241, issued Dec. 12, 1978, were both devoted to joining rotating anodes made from molybdenum and molybdenum-tungsten alloys to stems made from columbium and its alloys. Continuing increases in applied energy during tube operation have led to a change in target composition to titanium zirconium molybdenum (TZM) TZM is a trademark of Metalwork Plansee or other molybdenum alloys, to increased target diameter and weight, as well as to the use of graphite as a heat sink in the back of the target. Future computerized tomography (CT) scanners will be capable of decreasing scan time from a one second rotation to a 0.5 second rotation or lower. However, such a decrease in scan time will quite possibly require a modification of the current CT anode design. The current CT anode design comprises two disks, one of a high heat storage material such as graphite, and the second of a molybdenum alloy such as TZM. These two concentric disks are bonded together by means of a brazing process.
A thin layer of refractory metal such as tungsten or tungsten alloy is deposited to form a focal track. Such a composite substrate structure may weigh in excess of 4 kg.
With faster scanner rotation rates, heavy targets will increase not only mechanical stress on the bearing materials but also a focal spot sag motion causing image artifacts.
Furthermore, there is a demonstrated need for multi-energy or multiple target material sources of x-radiation. In mammography, for example, the image contrast is enhanced by using Mo and Rh target tracks with two separate electron beam sources. However, using two tracks with two electron beam sources increases mechanical complexity of high voltage, high power x-ray tubes due to the size of the resulting target and the consequent design choices that must be made: the size and mass of the rotor, stator, and certain features of the vacuum enclosure which act as the support frame. In addition, there are certain limitations to this design, for example, only two materials may be employed and two electron beam sources may be required, as in mammography. The large mass anode assembly makes changing target materials unfeasible or inconsistent with present design goals.
Accordingly, it would be desirable over the state of the art to provide a target anode structure and material which is capable of high speeds of rotation, and which is less sensitive to thermal stresses. It would also be desirable to provide a new method of creating a layer of x-ray emissive material on a target anode substrate which would not be subject to delamination. It would be desirable then to replace the present CT target design with a lightweight design comparable in thermal performance, particularly suited for use in x-ray rotating anode assemblies.
The above discussed and other drawbacks and deficiencies are overcome or alleviated by a rotatable anode for x-ray tube comprising: a solid thin plate target selected from a group of high-Z materials selectively deposited onto a substrate material including silicon, silicon carbide, aluminum nitride, gallium arsinide, glass or other commercially available thin disk substrate material. The substrate material includes single crystal, polycrystalline and amorphous forms. The plate target includes a substantially planar base surface extending from the axis of rotation to a periphery outlining the base surface, wherein the plate target includes target material for generating x-rays. The plate target has a thickness of about 1 mm or less.
In an alternative embodiment, a method for manufacturing a rotatable anode for an x-ray tube is disclosed. The method comprising: fabricating a thin plate target with silicon wafer processing technology using suitable materials for such technology in forming the plate target selected from a group of high-Z materials. The plate target includes an axis of rotation and a thickness of about 1 mm or less.
The above discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings.